Prism lighting is the use of prisms to improve the distribution of light in a space. It is usually used to distribute daylight, and is a form of anidolic lighting.
Prism lighting was popular from its introduction in the 1890s through to the 1930s, when cheap electric lights became commonplace and prism lighting became unfashionable. While mass production of prism lighting systems ended around 1940, [1] the 2010s have seen a revival using new materials. [2]
The human eye's response to light is non-linear: halving the light level does not halve the perceived brightness of a space, it makes it look only slightly dimmer. [3] [4] If light is redistributed from the brightest parts of a room to the dimmest, the room therefore appears brighter overall, and more space can be given a useful and comfortable level of illumination (see before and after images from an 1899 article, below). This can reduce the need for artificial lighting.
Refraction and total internal reflection inside optical prisms can bend beams of light. This bending of the light allows it to be redistributed.
Many small prisms may be joined at the edges into a sheet. A prism sheet is somewhat like a linear Fresnel lens, but each ridge may be identical. Unlike a Fresnel lens, the light is not intended to be focused, but used for anidolic lighting.
Deck prisms carried light through the upper decks of ships and spread it in the decks below. Similarly, on land, prisms in sidewalk lights were used to light basements and vaults.
Prism tiles were used vertically, usually as a transom light above a window or door. [6] They were also built into fixed and movable canopies, sloped glazing, and skylights. They bend light upwards, so that it penetrates more deeply into the room, rather than lighting the floor near the window. [5]
Modern prismatic panels are essentially an acrylic version of the old glass prism tiles. Like glass tiles, they can be mounted on adjustable canopies. [7] Channel panels use slits that reflect light internally. Holographic optical elements can also be used to redirect light. [8] [7]
Daylight redirecting window film (DRF) is a thin, flexible peel-and-stick sheet, with the optical layer generally made of acrylic. There are two types of film. Some film is moulded with tiny prisms, making a flexible peel-and-stick miniature prismatic panel. Other film is moulded with thin near-horizontal voids protruding into or through the acrylic; the slits reflect light hitting their top surfaces upwards. [9] [2] Refraction is minimized, to avoid colouring the light. [2] The reflection-based films are more transparent (both are translucent), but they tend to send the light up at the ceiling, not deeper into the room. Refraction-based films are translucent rather than transparent, but offer finer control over the direction of the outgoing light beam; the film can be made in a variety of prism shapes to refract light by a variety of angles.
Older glass elements were cast, and might be cut and polished. Prism tiles were often made of single prisms joined with zinc, lead, or electroglazed copper strips (rather like the methods used to join traditional European stained glass). [1] Sidewalk prisms were cast in one piece as single or multiple-prism lenses, and inserted into load-bearing frames. Daylight redirecting film is made of acrylic. [10]
Damaged prism tiles may be repaired, and as they came in standard designs, there is a salvage market in replacements. Replacements for one-piece castings can be commissioned. Weakened prism tiles may be reinforced with hidden bars, much like those used to reinforce stained glass. [1]
Sophisticated systems for lighting different sorts of spaces with prism tiles were developed. Generally, the goal was to send the available light across the room nearly horizontally. [5] One company sold tiles with nine prescriptions, giving different angles of refraction. [11] [5] Different prescriptions were often used in different parts of the same window transom, sometimes to disperse the light vertically, and sometimes to bend light horizontally around obstacles like pillars. [5]
Prism tiles sometimes have elaborate artistic designs moulded into the outside; Frank Lloyd Wright created over forty prism tile designs. [12]
Prism lighting works more effectively in light, open spaces. [5] Some believe that it contributed to the trend away from dark, subdivided Victorian interiors to open-plan, light-coloured ones.[ citation needed ] The removal or covering of old prism transom lights often leaves characteristically tall signage spaces over shop windows (see pictures).
Daylight redirecting window film was initially made of one redirecting film and one glare-reducing diffusing film, often located on different interior surfaces of a double-glazed window, [2] but integrated single films are now available. [13] Some daylight redirecting films reflect incoming light upwards off tiny near-horizontal reflectors, so at high sun angles they bend it sharply, throwing it upwards to the ceiling, where a typical ceiling diffuses the daylight somewhat deeper into the space. [9] Other daylight redirecting films refract light at any specified angle, ideally sending it nearly horizontally into the room. Redirecting films can be used as a substitute for opaque blinds. [2]
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomenon, but other waves such as sound waves and water waves also experience refraction. How much a wave is refracted is determined by the change in wave speed and the initial direction of wave propagation relative to the direction of change in speed.
An interference filter, dichroic filter, or thin-film filter is an optical filter that reflects some wavelengths (colors) of light and transmits others, with almost no absorption for all wavelengths of interest. An interference filter may be high-pass, low-pass, bandpass, or band-rejection. They are used in scientific applications, as well as in architectural and theatrical lighting.
A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent material, a sash set in a frame in the opening; the sash and frame are also referred to as a window. Many glazed windows may be opened, to allow ventilation, or closed to exclude inclement weather. Windows may have a latch or similar mechanism to lock the window shut or to hold it open by various amounts.
Daylighting is the practice of placing windows, skylights, other openings, and reflective surfaces so that direct or indirect sunlight can provide effective internal lighting. Particular attention is given to daylighting while designing a building when the aim is to maximize visual comfort or to reduce energy use. Energy savings can be achieved from the reduced use of artificial (electric) lighting or from passive solar heating. Artificial lighting energy use can be reduced by simply installing fewer electric lights where daylight is present or by automatically dimming or switching off electric lights in response to the presence of daylight – a process known as daylight harvesting.
Poly(methyl methacrylate) (PMMA) is the synthetic polymer derived from methyl methacrylate. It is used as an engineering plastic, and it is a transparent thermoplastic. PMMA is also known as acrylic, acrylic glass, as well as by the trade names and brands Crylux, Hesalite, Plexiglas, Acrylite, Lucite, and Perspex, among several others. This plastic is often used in sheet form as a lightweight or shatter-resistant alternative to glass. It can also be used as a casting resin, in inks and coatings, and for many other purposes.
An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are not prisms. The most familiar type of optical prism is the triangular prism, which has a triangular base and rectangular sides. Not all optical prisms are geometric prisms, and not all geometric prisms would count as an optical prism. Prisms can be made from any material that is transparent to the wavelengths for which they are designed. Typical materials include glass, acrylic and fluorite.
An optical coating is one or more thin layers of material deposited on an optical component such as a lens, prism or mirror, which alters the way in which the optic reflects and transmits light. These coatings have become a key technology in the field of optics. One type of optical coating is an anti-reflective coating, which reduces unwanted reflections from surfaces, and is commonly used on spectacle and camera lenses. Another type is the high-reflector coating, which can be used to produce mirrors that reflect greater than 99.99% of the light that falls on them. More complex optical coatings exhibit high reflection over some range of wavelengths, and anti-reflection over another range, allowing the production of dichroic thin-film filters.
Architectural glass is glass that is used as a building material. It is most typically used as transparent glazing material in the building envelope, including windows in the external walls. Glass is also used for internal partitions and as an architectural feature. When used in buildings, glass is often of a safety type, which include reinforced, toughened and laminated glasses.
A refractometer is a laboratory or field device for the measurement of an index of refraction (refractometry). The index of refraction is calculated from the observed refraction angle using Snell's law. For mixtures, the index of refraction then allows the concentration to be determined using mixing rules such as the Gladstone–Dale relation and Lorentz–Lorenz equation.
Light tubes are structures that transmit or distribute natural or artificial light for the purpose of illumination and are examples of optical waveguides.
In optics, a dispersive prism is an optical prism that is used to disperse light, that is, to separate light into its spectral components. Different wavelengths (colors) of light will be deflected by the prism at different angles. This is a result of the prism material's index of refraction varying with wavelength (dispersion). Generally, longer wavelengths (red) undergo a smaller deviation than shorter wavelengths (blue). The dispersion of white light into colors by a prism led Sir Isaac Newton to conclude that white light consisted of a mixture of different colors.
A light shelf is a horizontal surface that reflects daylight deep into a building. Light shelves are placed above eye-level and have high-reflectance upper surfaces, which reflect daylight onto the ceiling and deeper into the space.
A deck prism, or bullseye, is a prism inserted into the deck of a ship to provide light down below.
Anidolic lighting systems use anidolic optical components to light rooms. Light redirected by these systems does not converge to a focal point or form an image, hence the name.
Picture framing glass usually refers to flat glass or acrylic ("plexi") used for framing artwork and for presenting art objects in a display box.
With all solar thermal collector systems there is a potential risk that the solar collector may reach an equilibrium or stagnation temperature higher than the maximum safe operating temperature. Various measures are taken for optical overheating protection.
A skylight is a light-permitting structure or window, usually made of transparent or translucent glass, that forms all or part of the roof space of a building for daylighting and ventilation purposes.
Vaulted sidewalks, also called sidewalk vaults and areaways are sidewalks that are not placed directly on the ground. Rather, there is an empty space below them where the ground level used to be. This may happen where the street level has been raised over time, or where basements are extended, or as utility vaults.
Pavement lights (UK), vault lights (US), floor lights, or sidewalk prisms are flat-topped walk-on skylights, usually set into pavement (sidewalks) or floors to let sunlight into the space below. They often use anidolic lighting prisms to throw the light sideways under the building. They were developed in the 19th century, but declined in popularity with the advent of cheap electric lighting in the early 20th. Older cities and smaller centers around the world have, or once had, pavement lights. In the early 21st century, such lights are approximately a century old, although lights are being installed in some new construction.
Daylight redirecting film (DRF) is a thin, flexible plastic film which can be applied to a window to refract or reflect incoming light upwards so that the deeper parts of the room are lit more evenly. It can be used as a substitute for opaque blinds. It is a form of prism lighting.
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